If they are local companies or small businesses I would give them a follow up call or stop by. I have gotten a lot of donations by following up. I would wait until the month of your event because a lot of businesses will put the letters aside till the month where your event is.
Most of them get so many requests that they don't send a reply even if it is a no. When is your event and when did you send out requests? Usually you get more replies/ items in the month or two before the event.
I've sent out a ton of letters to companies seeking donations but have heard very little. Most are local companies that I do not have any personal connection with. Would it be appropriate to send a follow up email?